210 DYEING 



that most tissues of plants and animals are extremely unlike wool. 

 An acidic lake would be useless to us (see p. 223): our mordant 

 colours act as basic dyes, and are indispensable. 



The chemistry of mordanting in microtechnique is perhaps the 

 most perplexing of all the subjects with which this book deals. 

 The literature is scattered among journals seldom read by any 

 one person, and no serious attempt has ever previously been 

 made to integrate our knowledge of the subject into a compre- 

 hensible whole. The most convenient arrangement for the reader 

 would be to start with a detailed description of the w^hole process 

 of attaching one particular dye wdth one particular mordant, and a 

 facile story of this kind could indeed be written. In fact, however, 

 we lack the knowledge necessary for a consecutive account of the 

 whole process. Ionization of mordants has been best described 

 with the salts of chromium, lake-formation with those of alu- 

 minium, and the attachment of metal to tissue wdth ferric salts: 

 different dyes must be chosen to illustrate particular points. In 

 what follows no attempt will be made to push generalization too 

 far. It is better at the present time to disclose the gaps in our know- 

 ledge than to put forward a consistent theory too confidently. Certain 

 important facts and the rough outline of a synthesis will emerge. 



The mordants commonly used in microtechnique are salts, 

 especially sulphates, of iron, aluminium, and chromium. Double sul- 

 phates or alums are generally used. The use of alums rather than 

 simple sulphates is perhaps due in part to historical causes. Alums 

 are easily crystallized and therefore easily purified from the rocks 

 that contain them. Their use in dyeing has been known from rather 

 remote times. ^^ Until about the middle of the fifteenth century the 

 only rocks known to contain a suitable mordant were in Turkey; 

 but when Constantinople had been captured by the Turks in 

 1453, a fragment of the knowledge that escaped with the refugees 

 to the western world concerned the recognition of suitable rocks 

 and the mode of preparation of the mordant. Alum was first mined 

 in Great Britain about the end of the reign of Queen Elizabeth I. 



The alums generally used are these: — 



potassium alum, Al2(S04)3.K2S04.24H20; 

 ammonium alum, Al2(S04)3.(NH4)2S04.24H30; 

 iron alum, Fe2(S04)3.(NH4)2S04.24H20; 

 chrome alum, Cr2(S04)3.K2S04.24H20. 



